Aluminum nitride (AlN) has been recognized for many years as a semiconductor material with a sufficiently wide bandgap to attract attention as a suitable material for electronic devices, including photonic (“optoelectronic”) devices such as light emitting diodes (LEDs). As used herein, the term light emitting diode refers to a semiconductor device which emits visible light (i.e. the portion of the electromagnetic spectrum visible to the human eye) when a potential difference is applied, and a resulting electric current passes across a p-n junction in the device.
Aluminum nitride is of particular interest because of its extremely wide bandgap of about 6.32 electron volts (eV). Generally speaking, a semiconductor's bandgap represents the largest energy transition that can be produced by a junction device using that semiconductor. In turn, because of the well known relationship E=hu between the energy of a transition and the light that can be potentially emitted from it, semiconductor materials with wider bandgaps can produce higher energy transitions which in turn can produce light of higher frequency (υ) and shorter wavelength (λ).
When expressed in terms of visible light, for example, blue light is considered (somewhat arbitrarily) to fall within the wavelength range of 455 to 492 nanometers (nm) and thus requires a transition of between 3.67 eV and 3.97 eV. Many semiconductor materials have bandgaps that are generally smaller than this threshold, and thus cannot be used under any circumstance to produce blue light. In turn, because blue is one of the three primary colors (along with red and green), the difficulty in obtaining blue light from semiconductor LEDs translates into an accompanying difficulty of producing full color LED displays or LED white light sources, each of which require some contribution of blue light.
Aluminum nitride is also attractive for LEDs because it is a direct bandgap semiconductor; i.e. its radiative recombination process requires no phonon (vibrational energy) to conserve energy. Thus, AlN is expected to produce light more efficiently than do the indirect bandgap materials that produce phonons as well as photons in their transitions.
Blue light emitting diodes (including lasers and ultraviolet diodes and lasers), are not, however, the only attractive product that can be formed by wide bandgap materials. Wide bandgap materials such as aluminum nitride also tend to exhibit better physical and electronic stability at high temperatures, making them suitable for all sorts of electronic applications that occur at high temperature or under other such thermally stressed conditions.
Wide bandgap materials are also attractive for “power” electronic applications in which devices are required to amplify current and to otherwise handle high voltages and large currents. As known to those of ordinary skill in the art, as power increases across a junction, temperature resultingly increases as well until at a given temperature for a given material the diode characteristics will cease. Wide bandgap materials such as AlN accordingly offer advantages in such power devices.
Additionally, wider bandgap materials are radiation “hard”, meaning that they can better withstand the effects of bombardment with electromagnetic energy. This makes such materials attractive for military and other applications under which such bombardment would be expected.
Because most semiconductor devices, including LEDs, are p-n junction devices, however, obtaining p-type and n-type epitaxial layers of the material represents one of the necessary fundamental steps in the development of semiconductor materials into viable devices.
Aluminum nitride is a difficult material to work with. To date, the published reports of shallow (i.e. near the bandedge) level n and p-type doping of aluminum nitride have been vague and somewhat contradictory. Most of the interest in Group III nitride semiconductors has focused on various ternary and quaternary compounds such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), and indium aluminum gallium nitride (InAlGaN). The Group III nitrides are difficult to form into coherent bulk single crystals appropriate for substrates for semiconductor devices. Thus, the requirement that other materials (such as sapphire) serve as substrates for such devices has to some extent limited their development.
Furthermore, a functionally conductive layer requires an appropriate dopant. In previous work, silicon was used as a known dopant for gallium nitride but not for aluminum nitride. Silicon has been demonstrated to work as an n-type dopant for gallium nitride but has not performed appropriately as an n-type dopant for aluminum nitride in attempts to date. Doping of aluminum nitride has been particularly difficult to accomplish using the metal organic chemical vapor deposition technique (“MOCVD” or just “CVD”). Although the exact cause of such difficulty is unknown, and the inventors do not wish to be bound by any particular theory supporting the failure of others, CVD techniques generally use compound sources rather than elemental ones. For example, a typical aluminum CVD source is trimethylaluminum (“TMA,” (CH3)3Al); a typical nitrogen source is ammonia (NH3), and a typical germanium source is germane (GeH4). As a result, it appears that CVD techniques consistently tend to leave some amount of hydrogen (H2) in epitaxially grown films, and that the resulting hydrogen in the crystalline layer may interfere with the n-type doping in aluminum nitride. It has already been established that hydrogen affects p-type doping in gallium nitride, but interestingly enough hydrogen doesn't appear to affect silicon doping at all.
As another possibility, aluminum nitride has a tendency to scavenge oxygen (O2), which can likewise prevent successful doping. Such scavenging is more likely to occur in a CVD system and thus interfere with the overall doping process.